Re: Effect of a rs485 damaged driver

From: Reginald Jean Louis (louis_reginaldjean_at_hotmail.com)
Date: 09/23/04 

Date: Thu, 23 Sep 2004 15:42:50 -0400

I know I can detect bad frame, I just wanted to know if I can prevent a
damaged driver/receiver to corrupt the bus line.

"Tim Wescott" <tim@wescottnospamdesign.com> a écrit dans le message de
news:10l616hck5ocsa1@corp.supernews.com...
> Tim Shoppa wrote:
>
> > "Reginald Jean Louis" <louis_reginaldjean@hotmail.com> wrote in message
news:<9Os4d.21940$pA.1489170@news20.bellglobal.com>...
> >
> >>I have a bunch of rs485 transceivers arrange in a 4-wires bus topology
(my
> >>question is good for 2-wire/half duplex too). I want to know if a blown
up
> >>driver can affect the line by putting permanently the line in a high or
low
> >>state? If so, there is a way to prevent that?
> >
> >
> > Receivers can latch up/blow up too, for example by shorting one or both
> > of their inputs to Vcc or ground.
> >
> > The most common failure I've seen is shorted output drivers holding one
> > line high or low. Sometimes the system still continues to sort-of work
> > but not reliably at all.
> >
> > I've seen some applications guard against this by putting
50-ohm-or-greater
> > resistors in series between each node and the bus wires. Noise immunity
> > is decreased but the idea is that a "good" driver will outvote a "bad"
driver
> > that has only one of its outputs shorted to ground/Vcc. This doesn't
really
> > help if the "bad" driver is just ignoring its tristate input and
jabbering
> > all the time as it then (electrically) has just as much vote as a good
> > driver.
> >
> > Some bus networks have receivers that can issue an alarm if one of the
> > bus lines is stuck high/low or they see meaningless jabber. Haven't
> > seen this for RS-485 although some software will issue somewhat
meaningful
> > alarms when they cannot see their own transmissions :-).
> >
> > Tim.
>
> Asynchronous serial receivers can detect framing errors, and any
> protocol should include at least a checksum on messages -- you can alarm
> on either bad checksums, framing errors, or the line going silent.
>
> --
>
> Tim Wescott
> Wescott Design Services
> http://www.wescottdesign.com
>

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